1. Field of the Invention
The present invention relates to pulverulent binders containing specific oligo- and polyurethanes having (meth)acryloyl groups which may be applied to a substrate, melted by heating and cured by means of high energy radiation, especially UV radiation; to a process for their preparation; and to their use as a component of powder coating compositions.
2. Description of the Prior Art
Particularly high quality coatings from pulverulent binders are obtained using oligo or polyurethanes. Such compounds are described e.g. in EP-A 410242. They are prepared by reacting at least one organic polyisocyanate with at least one monohydric alcohol having (meth)acryloyl groups, and at least one isocyanate-reactive compound free from (meth)acryloyl groups.
These compounds must be prepared in an organic solvent. A solventless preparation is not possible due to the viscosities of urethane acrylates, which are known to be high at temperatures up to 120xc2x0 C. Increasing the preparation temperature beyond 100 to 120xc2x0 C. involves the risk, familiar to those skilled in the art, of a spontaneous undesirable polymerization of the compounds having (meth)acryloyl groups. Thus the disadvantage of the binders described in EP-A 410242 is that they must be prepared in an organic solvent, which has to be removed at high cost at the end of the preparation. Even small solvent residues strongly influence the melting behavior, which is critical to the application of the powder coatings.
To lower the melt viscosity of powder coatings based on unsaturated polyesters or polyacrylates with acryloyl functional groups, the powders are first prepared and then, according to EP-A 636 669, mixed with a low molecular weight crosslinking agent having vinyl ether, vinyl ester or (meth)acrylate functional groups. In the cited patent application, the polyacrylates with acryloyl functional groups and the crosslinking agent are also prepared in organic solvents, which subsequently have to be removed. Furthermore, mixtures of vinyl ethers or esters with unsaturated (meth)acrylates involve a considerably higher risk of an undesirable spontaneous polymerization (copolymerization of electron-rich and electron-deficient double bonds) than e.g. in the case of pure (meth)acrylate systems.
In Journal of Coatings Technology, vol. 70, no. 884, Sept. 1998, 57-62, A. Hult et al. describe radiation-curable powder coatings based on mixtures of amorphous polymers having methacrylate functional groups (polyacrylate) and crystalline (meth)acrylate monomers. Again, the polymers and monomers must be separately prepared using organic solvents, which have to be removed at high cost after the preparation.
An object of the present invention is to provide oligo- and polyurethanes having (meth)acryloyl groups, which are suitable as binders for powder coatings and which have a greatly reduced residual organic solvent content than prior art binders.
It has now been found that oligo- and polyurethanes having (meth)acryloyl groups can be prepared as binders for powder coatings without using solvents so that the residual organic solvent content is extremely low.
The present invention relates to pulverent binders for powder coating compositions which contain oligo- and polyurethanes having (meth)acryloyl groups, are curable by means of high energy radiation and have an organic solvent content of less than 1 wt. %.
The present invention also relates to a process for preparing pulverent binders for powder coating compositions containing oligo- and polyurethanes having (meth)acryloyl groups and curable by means of high energy radiation by reacting in the absence of an organic solvent
A) a monoisocyanate or diisocyanate having 4 to 20 carbon atoms and
B) a diisocyanate and/or a polyisocyanate that may be the same as A) or different from A) with
C) a monohydroxyalkyl (meth)acrylate having 2 to 12 carbons in the alkyl chain,
D) at least one alcohol having (meth)acryloyl groups that may be the same as C) or different from C) and
E) a compound which is free of (meth)acryloyl groups and contains two or more isocyanate-reactive groups,
wherein
i) the equivalents of OH groups in C) corresponds to the equivalents of isocyanate groups in A),
ii) the sum of the equivalents of OH groups in D) and the equivalents of isocyanate-reactive groups in E) corresponds to the equivalents of isocyanate groups in B) and
iii) the amount of the reaction product of A) and C), based on the total weight of oligo- and polyurethanes having (meth)acryloyl groups, is 10 to 95 wt. %.
The present invention further relates to the use of the binders in powder coating compositions for coating wood, metal, plastic, mineral and/or preprimed substrates.
The binders according to the present invention preferably contain 10 to 95 wt. % of the reaction product of component A) with component C) and 5 to 90 wt. % of the reaction product of component B) with components D) and E), wherein the percentages are based on the total weight of oligo- and polyurethanes having (meth)acryloyl groups. Preferably, the percentages of the reaction products add up to 100.
Suitable mono- or diisocyanates A) include aliphatic or aromatic isocyanates such as cyclohexyl isocyanate, butylene diisocyanate, hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), 3(4)-isocyanatomethylcyclohexyl isocyanate (IMCI), trimethylhexamethylene diisocyanate (2,2,4- and/or 2,4,4-trimethylhexamethylene diisocyanate), the isomeric bis(4,4xe2x80x2-isocyanatocyclohexyl)methanes, 1,4-cyclohexylene diisocyanate, phenyl isocyanate, toluyl isocyanate, 1,4-phenylene diisocyanate, 2,4- and/or 2,6-toluylene diisocyanate (TDI), 1,5-naphthylene diisocyanate, 2,4xe2x80x2- and/or 4,4xe2x80x2-diphenylmethane diisocyanate (MDI) and derivatives of the preceding isocyanates containing urethane, isocyanurate, allophanate, biuret, uretdione and/or iminooxadiazinedione groups, provided that these derivatives contain one or two free NCO groups. HDI, IPDI, TDI, MDI, the isomeric bis(4,4xe2x80x2-isocyanatocyclohexyl)-methanes, and mixtures thereof are preferred. IPDI, HDI and mixtures thereof are particularly preferred.
Component B) is selected from polyisocyanates having a functionality of at least 2. Suitable polyisocyanates include the known organic polyisocyanates from polyurethane chemistry, which have aliphatically, cycloaliphatically and/or aromatically bound isocyanate groups and a molecular weight of 144 to 1000, preferably 168 to 300. Examples include butylene diisocyanate, hexamethylene dilsocyanate (HDI), isophorone diisocyanate (IPDI), 3(4)-isocyanatomethylmethylcyclohexyl isocyanate (IMCI), trimethylhexamethylene diisocyanate (2,2,4- and/or 2,4,4-trimethylhexamethylene diisocyanate), the isomeric bis(4,4xe2x80x2-isocyanato-cyclohexyl)methanes, isocyanatomethyl-1,8-octane diisocyanate, 1,4-cyclohexylene diisocyanate, 1,4-phenylene diisocyanate, 2,4- and/or 2,6-toluylene diisocyanate (TDI), 1,5-naphthylene diisocyanate, 2,4xe2x80x2- and/or 4,4xe2x80x2-diphenylmethane diisocyanate (MDI), 4,4xe2x80x2,4xe2x80x3-triphenylmethane triisocyanate, derivatives of these polyisocyanates containing urethane, isocyanurate, allophanate, biuret, uretdione and/or iminooxadiazinedione groups, and mixtures of the preceding polyisocyanates. The derivatives preferably have a molecular weight of up to approx. 1000 and may be prepared in accordance with U.S. Pat. Nos. 3,124,605, 3,183,112, 3,919,218, 4,324,879 and EPA 798 299.
HDI, IPDI, TDI, MDI and/or polyisocyanates obtained by the trimerization of HDI, TDI or IPDI are preferably used as component B). HDI, IPDI and mixtures thereof are particularly preferred.
Monohydroxyalkyl (meth)acrylates C) are selected from hydroxyalkyl esters of acrylic acid or methacrylic acid having 2 to 12 carbon atoms, preferably 2 to 4 carbon atoms, in the hydroxyalkyl ester group. Examples include hydroxyethyl (meth)acrylate, 2- or 3-hydroxypropyl (meth)acrylate and 2-, 3- or 4-hydroxybutyl (meth)acrylate. 2-Hydroxyethyl acrylate is preferred.
The OH groups of C) are reacted with the NCO groups of component A) in approximately equivalent amounts, preferably in an OH:NCO equivalent ratio of 1:1.
Suitable alcohol components D) having (meth)acryloyl groups are acrylic acid or methacrylic acid esters of dihydric alcohols, which have a free hydroxyl group. Examples include 2-hydroxyethyl, 2- or 3-hydroxypropyl and 2-, 3- or 4-hydroxybutyl (meth)acrylate and mixtures thereof. Also suitable are monohydric alcohols having (meth)acryloyl groups or reaction products substantially containing these alcohols and obtained by esterifying n-hydric alcohols with (meth)acrylic acid. The alcohols used can also be mixtures of different alcohols, wherein n is an integer or a fractional number from 2 to 4, preferably 3. From (n-0.8) to (n-1.2) moles, preferably (n-1) mole, of (meth)acrylic acid is used per mol of said alcohols.
Examples include the reaction products of (i) glycerol, trimethylolpropane and/or pentaerythritol, low-molecular alkoxylation products of these alcohols (for example ethoxylated or propoxylated trimethylolpropane, such as the addition product of ethylene oxide and trimethylolpropane of OH number 550), and mixtures of these at least trihydric alcohols with dihydric alcohols (for example ethylene glycol or propylene glycol), with (ii) (meth)acrylic acid in the preceding molar ratio. The resulting reaction products have a molecular weight of 116 to 1000, preferably of 116 to 750 and more preferably of 116 to 158.
Preferably component D) is 2-hydroxyethyl acrylate.
Compounds E) are free from (meth)acryloyl groups; contain at least two, preferably 2 to 4 and more preferably 2 to 3 isocyanate-reactive groups, preferably alcoholic hydroxyl groups; and have a molecular weight of 62 to 200. Examples include ethylene glycol, 1,2- and 1,3-propanediol, neopentyl glycol, butanediol, hexanediol, glycerol, trimethylolethane, trimethylolpropane, pentaerythritol, sorbitol and di(trimethylpropane). Other suitable compounds, although less preferred, include the high molecular weight isocyanate-reactive compounds, especially polyols known from polyurethane chemistry. Examples include the known polyhydroxy polyethers or polyhydroxy polyesters having number average molecular weights above 200. However, the use of these high molecular weight compounds is only p possible in minor amounts because otherwise the melting range essential for powder coating binders cannot be met.
In a preferred embodiment polyester diols prepared from diols having a molecular weight of 62 to 400 and lactones are used completely or partially as component E). The preparation of such lactone polyesters is known per se. The molar ratios of diol to lactone are from 1:0.2 to 1:100, preferably from 1:0.5 to 1:5. Particularly preferred diols are 1,2-propanediol and 1,3- or 1,4-butanediol. The particularly preferred lactone is xcex5-caprolactone.
The sum of number of equivalents of isocyanate-reactive groups in components D) and E) corresponds to the number of equivalents of isocyanate groups in component B) in the process according to the invention.
The reaction of hydroxy-functional acrylates with isocyanates is known e.g. from P.K.T. Oldring (ed.), Chemistry and Technology of UV and EB Formulations For Coatings, Inks and Paints, vol. 2, 1991, SITA Technology, London, pp. 73-97.
In one embodiment for carrying out the process according to the invention isocyanate-containing components A) and B) are reacted with components C), D) and E), with the passage of an oxygen-containing gas, preferably air, at a temperature above the melting point of the product to be prepared (e.g., at a temperature of 30 to 140xc2x0 C.) until the NCO content has fallen below 0.1 wt. %. Component C), D) and E) can be charged successively and separately; simultaneously and separately; or as a mixture. All of a portion of component E) is preferably added last.
In another embodiment components C), D) and E) are reacted with isocyanates A) and B) under the preceding conditions. Again, components A) and B) can be charged simultaneously, successively, as a mixture or separately. If appropriate, not all of components C), D) and E) are initially added; a portion or all of component E) is not charged until the reaction is substantially complete.
The addition reaction to form the oligo- and polyurethane can be accelerated in a manner known by the addition of suitable catalysts. Examples include tin octoate, dibutyltin dilaurate or tertiary amines such as dimethylbenzylamine. The urethane acrylate obtained as the reaction product can be protected from premature and undesirable polymerization by the addition of suitable inhibitors and antioxidants, for example phenols and/or hydroquinones, in amounts of 0.001 to 0.3 wt. %, based on the polyurethane, in each case. These additives can be added before, simultaneously with and/or following the reaction to form the polyurethane.
The compounds can also be reacted in a tubular reactor, static mixer or reaction kneader. Compared with batch reactions in a reaction tank, very short reaction times ( less than 30 min) at elevated temperatures (80 to 200xc2x0 C.) may be used. In this process variant the reactants flow continuously, e.g. through a tubular reactor, and are reacted as they flow.
The polyurethanes or urethane acrylates obtained by the process according to the invention represent valuable binders for powder coating compositions. They are distinguished in particular by an extremely small amount of organic solvents. Products according to the invention have an organic solvent content of less than 1 wt. %, preferably less than 0.5 wt. %, more preferably less than 0.1 wt. % and most preferably less than 0.05 wt. %. They can be processed as thermally crosslinkable powder varnishes without further additives (in which case the binder would be identical to the coating composition) or, preferably, the known additives used in coating technology can be added. Examples include pigments such as titanium dioxide, flow control agents such as polybutyl acrylate or silicones, degassing agents such as benzoin. The additives are homogenized, e.g. on extruders or kneaders, at temperatures of approx. 40 to 140xc2x0 C., preferably 60-80xc2x0 C. Another alternative is to stir the additives into the melt as soon as the preparation has ended.
The solid obtained is then ground in a manner known per se and coarse particles, preferably those with a size greater than 0.1 mm, are removed by sieving.
The resulting pulverulent coating compositions can be applied to the shaped parts to be coated by conventional powder application processes, e.g. electrostatic powder spraying, triboelectric application or whirl sintering. The coatings are then initially melted heating (e.g. using IR radiation); a clear film forms, provided no pigments or flatting agents or the like have been incorporated. The temperature required for the powder to melt and flow is above the melting range of the coating composition. The coatings can be cured either by heating to a temperature of 130-220xc2x0 C., preferably 150-190xc2x0 C., and/or by the action of high energy radiation such as UV or electron beam radiation.
For crosslinking with UV radiation, photoinitiators have to be homogeneously incorporated into the coating compositions.
The known photoinitiators are suitable, provided they do not have an adverse effect on the powder properties such as flowability and storage stability. Examples include 1-hydroxycyclohexyl phenyl ketone, benzildimethylketal orxe2x80x94in the case of pigmented systemsxe2x80x942-methyl-1-(4-(methylthio)phenyl)-2-morpholinopropan-1-one or trimethylbenzoyl-diphenylphosphine oxide.
The photoinitiators are used in amounts of 0.1 to 10 wt. %, preferably of 0.1 to 5 wt. %, based on the weight of the polyurethanes. The photoinitiators can be used individually or, because of frequent advantageous synergistic effects, they can also be used in combination with one another.